Relative stability of soil carbon revealed by shifts in 15N and C:N ratio

1 Institute of Environmental Geosciences 2 School of Geosciences Edinburgh 3 Research Station ART

Abstract : Life on earth drives a continuous exchange of carbon between soils and the atmosphere. Some forms of soil carbon, or organic matter, are more stable and have a longer residence time in soil than others. Relative differences in stability have often been derived from shifts in ?13C which is bound to a vegetation change from C3 to C4 type or through 14C-dating which is bound to small sample numbers because of high measurement costs. Here, we propose a new concept based on the increase in ?15N and the decrease in C:N ratio with increasing stability. We tested the concept on grasslands at different elevations in the Swiss Alps. Depending on elevation and soil depth, it predicted mineral-associated organic carbon to be 3 to 73 times more stable than particulate organic carbon. Analysis of 14C-ages generally endorsed these predictions.